The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a system and method of driving a birdcage coil to generate an elliptically-shaped B1 field.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
It is generally desirable to have a relatively uniform sensitivity, both for signal generation and reception, throughout a cross-section of an imaged object when acquiring MR data. However, as the resonance frequency is increased, this becomes more difficult due to conductive losses and wavelength effects within the object. Further, as frequency increases, the power deposition required to achieve a given B1 field also increases.
Quadrature excitation and reception has been adapted as a standard method of operating volume RF coils to achieve a relatively uniform sensitivity or B1 field. Compared with a linear mode of operation, quadrature excitation and reception may result in signal-to-noise ratio (SNR) improvements of up to 40% and a power reduction of up to a factor of 2.0 for the same effective B1 field.
It is well known that in free space, a quadrature operated volume birdcage coil produces and receives a circularly polarized B1 field. For lossy objects with a relative permittivity similar to water and circular in cross-section, the B1 field is only truly circularly polarized over a small region at the center. Nevertheless, for a circular cross-sectioned object, a circularly polarized B1 field is considered the most efficient in terms of B1 field generated for a given amount of power.
For elliptically-shaped objects, however, circular polarization is less than optimal. That is, if a subject has an elliptical cross-section, then a circularly polarized B1 field is not well-matched to the subject and, therefore, is less than optimal. Similarly, the SNR on reception is less than optimal. Further, transmit efficiency, which is defined as B1 field strength divided by the square root of the power deposited in the subject, is relatively poor. This is particularly problematic in abdominal imaging and similar scans where the object under inspection is defined to have an elliptical cross-section.
It would therefore be desirable to have a system and method capable of generating a polarized B1 field better suited for an elliptically cross-sectioned subject.